ABSTRACT Lekhwair field is a carbonate formation located in the north of Oman and characterized by high brine salinity and temperature. In this study, an anionic surfactant (AES7-58) was selected to investigate its potentiality to be used in enhanced oil recovery (EOR) process for Lekhweir oilfield. Surfactant-brine chemical compatibility, surfactant-brine-oil phase behavior, and brine-oil interfacial tension (IFT) were examined in Lekhweir reservoir conditions (salinity and temperature). AES7-58 showed great tolerance to high salinity at reservoir temperature (70 °C). Phase behavior generated oil-in-water microemulsion (type I), in which low IFTs between the oil-in-water microemulsion and the excess oil were in the range from 3 x 10-2 mN/m to 6 x 10-2 mN/m. Low surfactant concentrations were considered for coreflood tests using Indiana Limestone core samples. As there is a little amount of oil left after the waterflooding, surfactant tertiary recovery ranged from 1-3% of original oil in place (OOIP), corresponding to 2.8-37.5% of residual oil in place (ROIP). Injection of 1PV of surfactant as a secondary recovery technique (88% OIIP) at 1000 ppm resulted in an accelerated oil recovery compared to waterflood (66-80% OIIP. (